An LED has advantages of long life, high light efficiency, and energy saving, so the LED is gradually utilized as a backlight source of an LCD. The following description refers to a conventional driving circuit of an LED backlight. Referring to FIG. 1, FIG. 1 is a schematic drawing illustrating a conventional driving circuit of the LED backlight. LEDs that are driven are only shown as “LOAD” in FIG. 1, and the load is the LEDs connected with a series connection or a plurality of serial LEDs connected with a parallel connection.
A first capacitor C1, an inductor L, a first switch Q1, a diode D, a first resistor R1, and a second capacitor C2, consist of external power components of a boost circuit. An output pin (pin 6) of a PWM (Pulse Width Modulation) chip (UC3843) is coupled to a gate of the first switch Q1. The PWM chip (UC3843) controls said external power components through the output pin, so that the boost circuit can supply the load with enough output voltage.
The first switch Q1, the second switch Q2, and third switch Q3, are MOSFETs, which can withstand voltage between the gate-source (GS) usually being positive and negative 20V. In order to prevent the switches to withstand too high of a voltage, a regulator 101 has to be disposed in the conventional boost circuit for supplying the first switch Q1 with a driving voltage and supplying the PWM chip (UC3843) with a VCC. The currently known regulators can be categorized into linear regulators and switching regulators. The linear regulators have a shortcoming of high power consumption as well as a requirement of a larger cooling area. The switching regulators, however, have higher power conversion efficiency than the linear regulators, but the switching regulators are expensive.